(Z)-2,3-Dichloro-1,4-bis(4-chlorophenyl)but-2-ene-1,4-dione

The title compound, C16H8Cl4O2, crystallizes with two independent molecules in the asymmetric unit. Both molecules have a Z conformation around the central double bond and they show significantly different C—C—C—O torsion angles between the aromatic ring and the carbonyl group [30.1 (7) and 3.9 (7)° in one molecule and 23.5 (7) and 9.3 (8)° in the other]. The crystal packing shows short halogen Cl⋯O [3.003 (5) and 3.246 (4) Å] and Cl⋯Cl [3.452 (2) Å] contacts and aromatic C—H⋯Cl and C—H⋯O interactions link the molecules, resulting in chains propogating along [100]. The crystal structure also features π–π stacking interactions between aromatic units of the two independent molecules, with a centroid–centroid distance of 3.9264 (6) Å.


S2. Structural commentary
Free radical reactions are intimately involved in the chemistry of trichloromethyl compounds. Generation of free radicals from trichloromethyl group by homolysis of a C-Cl bond is relatively easy. Free radicals can easily be generated by the action of UV-light, radical initiators or redox active metal salts or its complexes. Considerable amount of information is available in the literature on radical reactions involving trichloromethyl group containing compounds. For example, the radical generated by reaction of a trichloromethyl group substituted compound under non reducing condition with CuCl or its complexes with bpy or with other bi-or tridentate tertiary amine ligands readily undergo intermolecular (Martin et al., 1985) or intramolecular (Clark, 2002), (Ram & Kumar, 2008) addition/cyclization on to a suitably substituted carboncarbon double bond. The formation of mono-and/or di-reduction product are also reported under non reducing conditions along with cyclization products (Ram et al., 2007). Such radicals also acts as radical initiator in atom transfer radical polymerization reactions (Tomislav & Matyjaszewski, 2008), (Matyjaszewski & Xia , 2001). However, if the carboncarbon double bond in such radical centre is replaced by any weak or relatively better leaving group at the β-position of the radical centre, it underwent predominantly rearrangement and/or fragmentation by the intermediate formation of contact ion pair (Ram & Meher, 2003). It is worthwhile to mention that 2,2,2-trichloroethylalkyl ethers and trichloromethyl carbinols having no suitably located carbon-carbon double bond or a leaving group β-position to the trichloromethyl carbon underwent 1,2-H shift under similar conditions through the intermediacy of a copper-carbenoid species (Ram & Charles, 1999), (Ram & Manoj, 2008). In this context, we have decided to explore the behavior of the radicals derived from trichloromethyl compounds which neither contains any suitably located carbon-carbon double bond nor any leaving group or any hydrogen atom at the β-position of the radical centre so as to restrict the above transformations i.e. intermolecular or intramolecular addition; ATRP; rearrangement and/or fragmentation or 1,2-H shift. The major product obtained under such reaction conditions is reported here. The asymmetric unit ( Fig. 1) consists of the two formula units of the compound. Each formula unit adopts Z conformation about the C=C bond: C 8 =C 9 and C 24 =C 25 . The aromatic ring of two units are nearly coplanar with a dihedral angle of 12.73° (C 12 -C 15 -C 21 -C 18 ). A centroid to centroid distance of 3.9264 (6) Å between aromatic units of two independent molecules present in the asymmetric unit is observed indicating the presence of π-π stacking interactions (Fig. 1). The structure is stabilized by short intermolecular C-H···Cl

S5. Synthesis and crystallization
A two-neck round bottom flask fitted with a rubber septum was charged with CuCl (0.8 g, 0.008mol), 2,2′-bipyridine (1.25 g, 0.008 mol). Nitrogen was introduced into the flask followed by addition of 15 mL dry DCE or benzene into the flask to ensure the formation of the brown colored CuCl-bpy complex. To the reaction flask a solution of the 2,2,2-trichloro-1-(4-chloro-phenyl)-ethanone(0.004 mol) in dry DCE or benzene (5 mL) was added with the with the help of a syringe and the reaction mixture was heated to reflux with stirring under a slow and continuous flow of nitrogen. After the completion of the reaction as indicated by TLC (1-2 h), the reaction mixture was cooled and filtered through a celite pad. The filtrate was evaporated under reduced pressure on a rotary evaporator and purified by column chromatography using silica gel as the solid support. A solution of n-hexane and ethylacetate was used as the solvent for elution to get 1 in 52 or 60 % isolated yields in DCE or benzene respectively. Suitable crystals were obtained from chloroform/henxane.

S6. Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were placed at their ideal position with C-H = 0.93 A°.

Figure 1
The molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms showing π-π stacking interactions.  The packing diagram of the title compound showing short intermolecular halogen bond Cl···O interactions.

Figure 3
Structure of the title compound showing Cl···Cl, O···Cl, C-H···Cl and C-H···O interactions. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.31 e Å −3 Δρ min = −0.22 e Å −3 Absolute structure: Flack (1983), 1939 Friedel pairs Absolute structure parameter: 0.08 (7) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.